Image intensifier devices multiply the amount of incident light they receive and provide an increase in light output, which can be supplied either to a camera or directly to the eyes of a viewer. Image intensifiers are constructed for a variety of applications and hence vary in both shape and size. These devices are particularly useful for providing images from dark regions and have both industrial and military applications. For example, image intensifiers are used in night vision goggles for enhancing the night vision of aviators and other military personnel performing covert operations. They are employed in security cameras and in medical instruments to help alleviate conditions such as retinitis pigmentosis (night blindness).
As known, three major components of image intensifier tubes are the photocathode, phosphor screen (anode), and the MCP disposed between the photocathode and anode. These three components are positioned within an evacuated housing or vacuum envelope, thereby permitting electrons to flow from the photocathode through the MCP and the anode. In order for the image intensifier tube to operate, the photocathode and anode are normally coupled to an electric source, whereby the anode is maintained at a higher positive potential than the photocathode. Similarly, the MCP is biased and operates to increase the density of the electron emission set forth by the photocathode. Furthermore, since the photocathode, MCP and anode are all held at different electrical potentials, all three components are electrically isolated from one anther, when retained within the vacuum housing.
Referring to FIG. 1, there is shown a cross-sectional view of a conventional Gen III image intensifier tube 10 of the type manufactured by ITT Night Vision of Roanoke, Va. The prior art Gen III image intensifier tube 10 includes an evacuated housing 12 made from the assemblage of several separate components. Within housing 12, positioned are photocathode 14, microchannel plate (MCP) 16, and an inverting fiber optic element 18, the latter supporting phosphor screen 20. The construction for vacuum housing 12 forms an air tight envelope between photocathode 14 and fiber optic element 18.
The photocathode 14 rests upon a conductive support ring 22 at one end of vacuum housing 12. The abutment of photocathode 14 against support ring 22 creates an air tight seal, thereby closing one end of vacuum housing 12.
The lower end of vacuum housing 12 is sealed by the presence of an output screen flange 72. The output screen flange 72 is joined to fiber optic element 18 and forms an air tight envelope, thereby closing the other end of vacuum housing 12.
Between support ring 22, and screen flange 72 are additional elements providing annular spacers and electrical terminals for MCP 16 and fiber optic element 18. These elements are described in detail in U.S. Pat. No. 5,994,824, which is incorporated herein by reference in its entirety.
Completing the description of FIG. 1, an upper MCP terminal 32 extends into vacuum housing 12, where it conductively engages a metal hold down ring 36 and a contact ring 38. The contact ring 38 engages the conductive upper surface 42 of MCP 16, while the hold down ring retains the MCP within the housing. Consequently, an electrical bias may be applied to upper surface 42 of MCP 16 by applying the electrical bias to upper MCP terminal 32 on the exterior of vacuum housing 12. Similarly, a lower MCP terminal 48 extends into vacuum housing 12 and engages the lower conductive surface 44 of MCP 16. As such, the lower conductive surface 44 of MCP 16 may be coupled to ground by connecting the lower MCP terminal 48 to a ground potential external of vacuum housing 12.
Referring next to FIGS. 2 and 3, two examples are shown of how MCP 16 is fixed into position and sandwiched between an upper electrical terminal contacting upper MCP surface 42 and a lower electrical terminal contacting lower MCP surface 44. In FIG. 2, a ceramic ring 78 is positioned below support ring 22 (FIG. 1), and joined to the support ring during a brazing operation. The brazing operation creates an impervious seal between support ring 22 and ceramic ring 78. The ceramic ring 78 is part of ring assembly 86 for retaining the MCP.
The ring assembly 86 includes ceramic ring 78, conductive snap ring 77, MCP ceramic ring 46, and MCP lower support terminal 48. The ceramic ring 78 includes a first metalized surface 88 in electrical contact with conductive snap ring 77, and a second metalized surface 89 for providing electrical contact external to the housing and permit an electric source to be applied. The conductive snap ring 77 is formed of a metal or a metallic alloy. Snap ring 77 has a surface 77B conductively engaging the upper surface 42 of the MCP, and another surface 77A bonded to surface 88 of ceramic ring 78.
As illustrated, the conducting snap ring 77 is positioned between ceramic ring 78 and the MCP upper surface 42. The MCP rests against and is retained by snap ring 77 and ceramic ring 78. The MCP insulator ceramic ring 46 is positioned below and coupled to metalized surface 89 by a brazing ring (not shown) interposed between the two elements. The MCP insulator ceramic ring 46 is brazed to both metalized surface 89 and MCP lower support 48.
Thus, snap ring assembly 86 retains the MCP by using metalized ceramic 78 in combination with metalized snap ring 77 to provide both the lockdown and electrical contact. This feature eliminates the need for complex metal parts including mechanical rings and tabs used in other image intensifiers to hold the MCP in a fixed position.
In another example, as shown in FIG. 3, the MCP lower support 48 is employed to both laterally center and axially support MCP 16. The lower support structure provides a tab portion 48A, which is disposed laterally to surface 16C of the MCP, to prevent lateral dislocation of the MCP, and at the same time to maintain sufficient distance from the snap ring conductive surface in order to prevent short circuiting the device. The upper support structure 32, on the other hand, is curved downwardly toward snap ring 77 at its end portion 32A. The spring force of snap ring 77 is effective in forming a normal force against end portion 32A, so that the snap ring becomes wedged between the MCP and the upper support structure 32. In this manner, the upper surface 42 of the MCP is provided with an electrical potential by way of both the snap ring and the upper support structure. In addition, the MCP is fixed and locked down into position within the image intensifier tube.
A top view of snap ring 77 is shown in FIG. 4. In the example shown, the outer diameter 77G of the snap ring is 1.3 inches, whereas the ceramic ring 78 (FIG. 2) has an aperture with an inner diameter of 1.24 inches (for example). As a result, the snap ring includes dual cavities 77F, which are carved out of the ring's surface, for compressing the ring with the help of pliers. After compressing its outer diameter, snap ring 77 fits into the recess formed by the chamfer of metalized surface 88. In this manner, snap ring 77, at surface 77A, conductively engages metalized surface 88. In addition, at surface 77B, the snap ring conductively engages the upper surface 42 of MCP 16. The snap ring also secures and locks down the MCP into a fixed position within the housing.
The above described method of securing the MCP with the snap ring results in some drawbacks. One drawback is the extra effort required to compress the snap ring with pliers, and properly release the compression after the snap ring is placed on top of the MCP. Another drawback is the possibility of cracking the MCP, when the compression of the snap ring is unevenly released. The present invention, as will be explained, provides a solution to these drawbacks.